1. Field of the Invention
This invention relates generally to electronic circuitry operable at millimeter wave frequencies, and more particularly to a transition for efficiently joining a coaxial transmission line to a microstrip transmission line.
II. Discussion of the Prior Art
In applications requiring a high degree of microminiaturization, operation at millimeter wavelength frequencies allows tiny active and passive components to be employed. For example, only a small length of printed wiring may exhibit a capacitive reactance comparable to that provided by a ceramic capacitor operating at lower frequencies.
One mode of transmitting these RF signals is the so-called microstrip transmission line. It comprises a conductive strip of a relatively narrow width on one major surface of a dielectric planar substrate and a relatively wide conductive ground plane disposed on the opposite major surface of the dielectric substrate. High frequency signals in the millimeter wavelength range propagate along the surface of the microstrip line. Such microstrip transmission lines may be exposed, but oftentimes will be contained within a conductive box-like enclosure or housing to prohibit stray radiation emanating from the transmission line to deleteriously affect other components within the electronic system embodying the microstrip transmission line. Where a closed system is utilized, it is necessary to employ a feed-through to bring the transmitted signal in and out of the enclosure. Such a feed-through may typically be a length of coaxial transmission line. Even with open or uncased microstrip transmission lines, it is still necessary to provide a means for coupling the signal output from that medium to a load such as, for example, an antenna. Again, this necessitates a transition device for mating the microstrip media to the coaxial line media.
With uncased microstrip transmission lines, practice in the past has been to utilize an edge transition in which the outer conductive shield and the center insulation of the coaxial line is stripped back to expose the center conductor and then that center conductor is laid upon the microstrip conductor and a solder bond is used to join the two. Similarly, the outer solid tubular conductive shield of the coaxial line is conductively joined to the ground plane of the microstrip media, again using solder. This type of edge-launch transition is not altogether satisfactory in many applications, primarily because of packaging limitations, weakness of the joint and space considerations. For example, the physical geometry of the particular electronics package may preclude the use of an edge-launch transition between the microstrip and the coax line.
When the microstrip transmission line is contained within a conductive housing, the ground plane of the microstrip transmission line assembly can be soldered to the base or floor of the housing and, likewise, the tubular conductive shield of the coaxial line may also be soldered to the housing. This results in a stronger, more rugged construction but, as pointed out below, physical constraints of the electronic package itself may preclude this type of edge-launch transition.
For the foregoing reasons, a need exists for a more efficient means of joining a microstrip transmission line to a coaxial transmission line in a side-launch configuration. The present invention fulfills such a need. In particular, the present invention provides a means for coupling microstrip devices or systems to other transmission media, coaxial line in this case. In accordance with the present invention, a side-launch transition has been devised. To the best of our knowledge, we are the first to devise a side-launched coax to microstrip transition operable at millimeter wave frequencies. It is our belief that no one has earlier attempted a side-launch transition at millimeter wave frequencies because of the difficulty in simultaneously converting from coaxial mode to microstrip mode and in compensating for the large parasitic reactances present at millimeter wave frequencies.